7.  Chemical Compounds in the Body.  We must keep in mind that, with slight exceptions, none of these 13 elements exist in their elementary form in the animal economy.  They are combined in various proportions, the results differing widely from the elements of which they consist.  Oxygen and hydrogen unite to form water, and water forms more than 2/3 of the weight of the whole body.  In all the fluids of the body, water acts as a solvent, and by this means alone the circulation of nutrient material is possible.  All the various processes of secretion and nutrition depend on the presence of water for their activities.

8.  Inorganic Salts.  A large number of the elements of the body unite one with another by chemical affinity and form inorganic salts.  Thus sodium and chlorine unite and form chloride of sodium, or common salt.  This is found in all the tissues and fluids, and is one of the most important inorganic salts the body contains.  It is absolutely necessary for continued existence.  By a combination of phosphorus with sodium, potassium, calcium, and magnesium, the various phosphates are formed.

The phosphates of lime and soda are the most abundant of the salts of the body.  They form more than half the material of the bones, are found in the teeth and in other solids and in the fluids of the body.  The special place of iron is in the coloring matter of the blood.  Its various salts are traced in the ash of bones, in muscles, and in many other tissues and fluids.  These compounds, forming salts or mineral matters that exist in the body, are estimated to amount to about 6 per cent of the entire weight.

9.  Organic Compounds.  Besides the inorganic materials, there exists in the human body a series of compound substances formed of the union of the elements just described, but which require the agency of living structures.  They are built up from the elements by plants, and are called organic.  Human beings and the lower animals take the organized materials they require, and build them up in their own bodies into still more highly organized forms.

The organic compounds found in the body are usually divided into three great classes: 

  1.  Proteids, or albuminous substances.
  2.  Carbohydrates (starches, sugars, and gums).
  3.  Fats.

The extent to which these three great classes of organic materials of the body exist in the animal and vegetable kingdoms, and are utilized for the food of man, will be discussed in the chapter on food (Chapter V.).  The Proteids, because they contain the element nitrogen and the others do not, are frequently called nitrogenous, and the other two are known as non-nitrogenous substances.  The proteids, the type of which is egg albumen, or the white of egg, are found in muscle and nerve, in glands, in blood, and in nearly all the fluids of the body.  A human body is estimated to yield on an average about 18 per cent of albuminous substances.  In the succeeding chapters we shall have occasion to refer to various and allied forms of proteids as they exist in muscle (myosin), coagulated blood (fibrin), and bones (gelatin).